Porous substrates are available for various filtration and separation processes. For example, porous substrates with catalytic materials deposited on the substrate are commonly used to reduce particulate emissions and convert toxic exhaust gas into less toxic gases. In certain applications, the chemical conversion is also a useful step in synthesis of intermediate or final compounds. Substrates that have relatively high porosity (i.e., percentage of void space in the material) and relatively high thermal shock resistance (e.g., due to low thermal expansion) may provide the greatest efficiency and effectiveness.
Porosity is generally defined as the percentage of void space in a material. For example, in a substrate with 50% porosity, half of the volume of the substrate is void or open space within the substrate material. Higher porosity in a substrate corresponds to a lower mass per volume in the substrate, which is beneficial when the application requires an elevated operational temperature. For example, when a substrate is used in a catalytic process requiring a high operational temperature, a substrate with a low thermal mass will heat to the operational temperature faster than a substrate with a higher thermal mass, resulting in a shorter light off time for the catalyst.
While porosity is important for filtration and emissions control substrates, even a highly porous substrate is ineffective as a filter where gases have to flow through the filtration medium if it isn't also highly permeable. Permeability is generally defined as the measure of the ability of a material to transmit fluids. For example, in an emissions application, a highly porous substrate cannot effectively filter and convert the exhaust from a vehicle if the exhaust gas cannot flow through the substrate. Thus, it is important for the pores to be interconnected in order to obtain optimal flow through.
Substrates used as a support for catalytic reactions are typically coated with a washcoat, or a high surface area carrier coating, which may be subsequently catalyzed through the addition or impregnation of precious metals or catalytic materials. The washcoat provides high surface area for the dispersion of and stabilization of catalytic materials. In honeycomb substrates, such as the type typically used in exhaust emission controls, the washcoat is deposited over the entire wall of the honeycomb channels. In this flow-through configuration, the limitations on the amount of washcoat coating are dictated by the backpressure resulting from channel size reduction. In a porous substrate, particularly when configured in a wall-flow configuration as a filter, the washcoat limitations are dictated by the backpressure resulting from a reduction in porosity and permeability as the washcoat materials fill the void space within the porous substrate.
The washcoat materials are typically applied through the use of an aqueous slurry of a colloidal suspension of the washcoat materials, such as alumina powder and/or other refractory oxides, or from solution based methods. In the case of a slurry process, the washcoat materials are dispersed in an acidified water-based solution, and mixed using a high shear mixing process. The particle size of the washcoat materials must be carefully controlled to ensure proper adhesion and penetration when applied to the substrate material, and the viscosity of the slurry must be carefully controlled. The slurry is applied to the substrate, typically by pouring the solution into the substrate, which is then dried and calcined. In the case of a solution process, washcoat components, usually in the form of soluble salts in an aqueous solution that are applied to the substrate, dried and then calcined.
Extrusion of ceramic powder materials, and subsequent washcoat loading has proven to be an effective and cost efficient method of producing ceramic substrates for the environmental controls industry. However, there is an upper limit to the porosity in extruded ceramic powder materials that, if exceeded, results in low strength and decreased functionality. Further, porosity of a fired substrate may be reduced in post-production catalyst deposition, in which a washcoat, or surface enhancer, and/or precious metal catalyst material is applied to the finished substrate, potentially filling in voids, or pores, in the substrate.
In addition, the deposition of the washcoat to a fired ceramic honeycomb substrate adds an extra step in the processing and increases the cost of the washcoated substrate. Often, when a high washcoat loading is required, multiple washcoat processing steps have to be taken, which increase the cost and reduce uniformity of the washcoat loading.
Thus, there exists a need for a high-porosity filter substrate in which the washcoat and/or catalyst is included during production.